Behaviour of bromine in Cl- and F-bearing alkali-rich felsic magmas at crustal depth: An experimental study at 800–1100 °C, 10–200 MPa

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Bromine, although a minor component in volcanic gases, has received increasing interest in recent studies due to its high atmospheric ozone depletion potential, but its behaviour in alkali-rich felsic hydrous magmas remains unexplored. In this study, fluid-melt partitioning experiments were carried out using natural, Cl- and F-bearing silicate glasses with phonolitic, comenditic and pantelleritic compositions. For each composition, experiments were performed with a range of Br concentrations, at P-T conditions simulating isothermal decompression degassing or isobaric equilibrium cooling at shallow crustal depth (800–1000 °C, 10–200 MPa, oxidising and reducing conditions). The major element, Cl and F concentrations of the run-product glasses were determined by electron microprobe and the Br concentrations by LA-ICPMS. Volatile concentrations in the fluid were determined by mass balance calculations. The experimental results show that more Br partitions into the fluid phase with increasing bulk halogen concentration. The most Br-doped experiments are typically saturated with a vapor phase and a hydrosaline liquid. Experiments at the lowest Br concentrations, closest to natural systems, show that the pressure dependence of vapor-melt Br partitioning is complex, with a minimum vapor-melt partition coefficient observed at 50 MPa in the phonolitic composition (1.8 ± 0.9 at 1000 °C and oxidising conditions). Our results indicate that the eruptive degassing of Br from peralkaline felsic magmas is restricted to the shallowest levels of the magmatic plumbing system and is likely to occur at much lower pressures than in metaluminous magmas. This observation is consistent with the composition of melt inclusions preserved in alkaline silicic magmas. An important finding is that the vapor-melt partitioning of Br and Cl decreases with increasing temperature. In phonolite, at 200 MPa and oxidising conditions, the Br vapor-melt partition coefficient decreases from 18.5 ± 3.6 at 900 °C to 3.8 ± 1.5 at 1000 °C. As the ratio between the Br and Cl vapor-melt partition coefficients is not conservative, the Br/Cl ratio in the vapor phase is likely to increase during isobaric cooling and degassing. The vapor-melt partition coefficients of Br and, to a lesser extent Cl also increase with decreasing fO2 in the phonolitic system, and the Br vapor-melt partition coefficient for a reduced alkaline magma is close to that for an oxidised calc-alkaline magma. Our results also show that during protracted storage at shallow levels, oxidised alkali- and F-rich rhyolites coexist with vapor and brine. This suggests that high F concentration promotes unmixing of the halogen-bearing phase coexisting with such melts. The exsolution of immiscible vapor and brine efficiently removes Br from peralkaline magmas and probably limits the flux of Br to the atmosphere from such magmas.